We are interested in the safety mechanisms that prevent chromosome translocations that lead to the generation of lymphomas and leukemias, which are among the most common human cancers. With millions of lymphocytes undergoing V(D)J recombination each day, there are many opportunities for genomic integrity to be breached over the course of an individual's lifetime-- and chromosomal translocations are, in fact, a cardinal feature of lymphoid neoplasms. These rearrangements typically place an intact proto-oncogene under the regulatory control of the highly expressed Ig or TCR genes, leading to deleterious effects on cell growth, differentiation, or apoptosis. Compelling circumstantial evidence has implicated the V(D)J recombinase in these translocations, but how it might actually participate in aberrant rearrangements has remained mysterious. The Roth laboratory has recently made several discoveries that suggest novel mechanisms for V(D)J recombination-initiated aberrant rearrangements. We have also uncovered a new control point in the reaction that may prevent these events. The overarching goal of the studies proposed here is to understand the mechanisms that preserve genomic stability in lymphocytes and to define the molecular pathogenesis of V(D)J recombination-associated oncogenic rearrangements. To do so we will 1) probe the nature of the post-cleavage complex's scaffolding role; 2) examine the contribution of alternative RAG cleavage mechanisms to aberrant rearrangements; 3) create two lines of knock-in mice bearing RAG mutations we predict will increase the likelihood of aberrant rearrangements; 4) delineate the specific contributions of DNA damage sensors, checkpoint functions and the V(D)J recombinase to maintaining genomic stability. The last aim is in collaboration with the Petrini lab and capitalizes on their recent work implicating the Mrel I complex in controlling the fidelity of V(D)J recombination.